1. Field of the Invention
The present invention relates to distance measuring devices.
2. Description of the Related Art
It is often necessary to measure a distance between two measurement points such as from a first surface to another surface. For example, in order to improve oil and gas drilling and production operations, it is necessary to gather as much information as possible on the properties of the underground earth formation as well as the environment in which drilling takes place. Such properties include characteristics of the earth formations traversed by a well borehole, in addition to data on the size and configuration of the borehole itself. Among the characteristics of the earth formation measured are the resistivity, the density, and the porosity of the formation. However, the processes often employed to measure these characteristics are subject to significant errors unless information on the borehole size and configuration is also taken into account in their determination. Knowledge of the borehole size is also useful to estimate the hole volume, which is then used to estimate the volume of cement needed for setting casing or when hole stability is of concern during drilling.
The collection of downhole information, also referred to as logging, is realized in different ways. A well tool, comprising transmitting and detecting devices for measuring various parameters, can be lowered into the borehole on the end of a tubing, cable, or wireline. Parameter data measured by the tool is sent up to the surface using a cable attached to a mobile processing center at the surface. With this type of wireline logging, it becomes possible to measure borehole and formation parameters as a function of depth, i.e., while the tool is being pulled uphole.
It is known in the art to measure the diameter, also known as the caliper, of a borehole to correct formation measurements that are sensitive to size or standoff. These corrections are necessary for accurate formation evaluation. One technique for measuring the caliper incorporates a mechanical apparatus with extending contact arms that are forced against the wall of the borehole. However, this technique has practical limitations because of the mechanical instability of the caliper arms.
Due to the unsuitability of mechanical calipers to drilling operations, indirect techniques of determining borehole calipers have been proposed. Conventional caliper measurement techniques include acoustic transducers that transmit ultrasonic signals to the borehole wall. However, the techniques proposed with acoustic calipers entail measurements employing standoff and travel time calculations, resulting in data of limited accuracy. Sound wave reflections in soft formations may also be too weak to be accurately detected, leading to loss of signals.
Measuring the diameter of a borehole is only one of an unlimited number of examples where distance needs to be measured. It is desirable to obtain a simplified method and system for accurately determining a distance. Still further, it is desired to implement a distance measurement technique that is capable of measuring a wide range of distances.
The present invention overcomes the deficiencies of the prior art.